1. Field of the Invention
This invention relates to an active filter that has a tunable frequency response.
2. Description of the Prior Art
In the past several years, several filter designs have emerged that can be largely or entirely integrated onto a single semiconductor chip. Such filters are increasingly useful in telecommunications applications wherein it is necessary to provide filtering for voice or data channels, and numerous other applications. Filters are also widely used for testing and measuring, instrumentation, signal recovery, etc. Among the technologies that have been considered for integrated filters include charge-coupled devices (CCD) and switched capacitor filters. In addition, nonswitched filtering has also been investigated. In some respects, the nonswitched circuits are preferable, as none of the problems related to sampling the desired signal are present. In addition, the circuits are not limited by the maximum sampling rate at which the switched capacitor can be operated.
In order to obtain highly accurate time constants, it is possible to utilize a stable clock waveform for the adjustment of circuit time constants; see, for example, "Fully Integrated Analog Filters Using Bipolar-JFET Technology," by K. S. Tan et al in IEEE Journal of Solid-State Circuits, Vol. SC-13, pages 814-821 (1978). In that technique, an adjustable time constant is obtained by utilizing a voltage-controlled element. By comparing the time constant of one such tunable circuit on an integrated circuit chip to the reference clock, corrective feedback can be used to adjust the value of a multiplicity of other time constants. This is because on a single integrated circuit chip, the repeatability of devices from one tunable circuit to another is good. However, this technique relies upon the availability of a control element that can be conveniently integrated on a chip and which allows for a suitable range of time constants to be obtained. One prior art control element has been a differential pair of field effect transistors operated in a variable transconductance mode that responds to a control voltage; see the above-noted article by K. S. Tan et al at FIG. 6, etc.
In a voltage-controlled time-constant circuit, it is desirable to obtain a control element that can be implemented as simply as possible. It is also desirable that all active devices be implemented using a single technology, such as MOS technology, rather than using mixed device technology, such as both bipolar and MOS, on a single chip. In addition, it is necessary that the control circuitry obtain a high degree of linearity to obtain suitably low distortion. This allows, for example, signals having widely varying amplitudes to be filtered without excessive distortion or noise. An additional requirement for a useful implementation of a tunable filter is that the control element be readily integrated, preferably using standard semiconductor device processing techniques.